Most mobile radio standards specify that the output power of the mobile radio must be variable. In this case, the base station arranges the level at which the mobile radio must transmit as a function of the reception conditions.
In this way, the output level of the mobile radio can be varied between 5 dBm and 27 dBm in the low band (GSM850 and GSM900) for the GSM EDGE (GSM Enhanced Data Rate for GSM Evolution) standard, while the values for the output level in the high band (DCS1800 and PCS1900) are between 0 dBm and 26 dBm. A dynamic range of 22 dB thus results just for power adjustment in the low band and a dynamic range of 26 dB results in the high band. Future mobile radio standards such as UMTS (Universal Mobile Telecommunications System) will be subject to even more stringent requirements.
The GSM EDGE Standard uses modified 8PSK modulation as the modulation type, which allows a transmission rate which is three times higher than that of GMSK modulation. In this case, in contrast to GMSK modulation, the modulation data is subjected not only to phase modulation but also to amplitude modulation, and this necessitates a linear transmission chain. When using a linear power output stage, the output power of the mobile radio can then be controlled by varying the output amplitude of the transmitter module.
A robust transmitter structure, which is highly efficient because of large-scale digitization for the implementation, is used for so-called polar modulation. This is based on the idea that any radio-frequency signal R(t), modulated in any desired way, can be represented in the following polar coordinate form:R(t)=A(t)·cos(ωt+φ(t)),  (1)where A(t) denotes the amplitude information which varies over time, φ(t) denotes the phase information which varies over time, ω denotes the circular frequency of the radio-frequency oscillation and t denotes the time. The payload information, such as speech or data, is contained in the amplitude A(t) and in the phase φ(t).
In order to implement the polar modulation scheme at the transmitter end, the phase φ(t) is transformed in the transceiver module by means of a phase locked loop to the radio-frequency level (corresponding to the cosine term in equation (1)). The amplitude information A(t) is then applied in an amplitude modulator, for example a mixer. The magnitude of the output signal from a polar modulator such as this can be varied in a suitable manner by means of an amplifier. This can be carried out on the one hand in an analogue form by a control voltage, which is applied to a VGA (Voltage Gain Amplifier) from the baseband module. On the other hand, in principle, the same functionality can be achieved by the use of an amplifier with a digitally programmable gain, a so-called PGA (Programmable Gain Amplifier), in which case the gain of a programmable amplifier is set by means of a digital control word. However, in the case of a digitally programmable amplifier, the overall architecture must be adapted such that it is ensured that the power is raised continuously from zero to the intended value at the start of a burst, while it is ensured that it is reduced in the same way at the end of the burst. This raising and reducing of the power is generally referred to in the specialist literature as “power ramping”.
Both in the case of analogue and digital gain adjustment, the output voltage from the amplifier is supplied to a power output stage, which amplifies the radio-frequency signal to the desired level at the antenna.
The choice as to whether preference is given to the use of an analogue or a digital variable amplifier solution, depends on the constraints, such as the transmission standard, the technology, the drive capabilities in baseband, the transmission purity required and the power consumption. Irrespective of the choice of the amplifier, it should be remembered that the overall gain tolerance of the system in a linear chain is composed of the gain tolerances of the individual modules in the transmission path. These gain tolerances in turn depend on the temperature and frequency responses, on the operating voltage and on ageing. In order to obtain the required output power, the gain tolerances of the individual modules in the transmission path must be compensated for by means of the analogue or digital variable amplifier. This results in widening of the required dynamic adjustment range of the amplifier well beyond the range mentioned above. For example, this necessitates more than 40 dB in the high band for the GSM EDGE Standard. Furthermore, particularly for the very high output power levels, very stringent requirements are specified for the level accuracy by the 3GPP Specification and the network operators. The gain of all the components in the transmission path may therefore fluctuate only slightly in terms of the parameters mentioned above and must be reproducible, at least to the extent that it can be compensated for by software means in the baseband section of the transmission path.
In the case of previous linear GSM EDGE transmitter architectures, the amplifier can generally be varied in an analogue form, and is arranged in the radio-frequency section of the transmission path. This solution has the disadvantage of the high power consumption of the VGA module, which is required for high spectral purity of the output signal and for low noise. If CMOS (Complementary Metal Oxide Semiconductor Circuits) technology is used rather than the bipolar technology that is conventionally used for design of the amplifier in the course of changing over to modern, low-cost solutions, then this results in considerable linearity problems more particularly in downward control of the amplifier, which can be overcome, if at all, only by using large quiescent currents. Furthermore, one is confronted with the fact that the tolerances are wider than in the case of bipolar solutions, making it more difficult to trim the output power during the production of the mobile radio.
Although, as far as the Applicants are aware, this has not been achieved so far, in particular, the analogue, variable amplifier could also in principle be arranged in the baseband section of the transmission path. This would lead to a simplification of the circuit development for a linear amplifier concept and would lead to power consumption savings. One major disadvantage of a solution such as this would, however, be the occurrence of offset voltages in the amplifier and in other circuit blocks, which would result in severe carrier breakthrough but particularly when using the amplifier to reduce the wanted level. In the case of GSM EDGE, this would contravene the OOS (Origin Offset Suppression) requirement for direct modulator systems, so that, in general, the quality of the transmission signal would be reduced. In the case of polar modulator systems, breakthrough of the phase-modulated carrier would cause severe adjacent channel distortion. High-precision trimming of offset voltages such as these is possible, if at all, only with major effort.